1. Field of the Invention
This invention relates to the radiation therapy applied to patients who are typically treated for cancer or other ailments with high energy x-rays from medical linear accelerators. The invention is directed to a method and system to reconstruct the dose the patient has received using as input the radiation fields measured during treatment on the exit side of the patient with an imaging system capable of integration or a radiation detector array. This information may be used as a quality control measure of the treatment to avoid errors by comparing the reconstructed dose to the intended planned dose.
2. Prior Art
U.S. Pat. No. 5,754,622 describes a system for verifying the amount of radiation delivered to an object. However, this system does not describe the method to accomplish a reverse calculation. In particular, there are several problems not addressed. Radiation absorbed in the patient will cause scatter radiation, some of which will strike the imaging device (3) adding to the signal recorded. Some imaging devices are very energy dependent and will over respond to the scattered radiation. The spectrum of the x-rays traversing through the patient will change as some energies of the spectrum are differentially absorbed and scatter by the patient more than other energies in the spectrum. The imaging device (3) may respond differently to the rays (4) having traversed through different thicknesses of the body (22) because of the change in spectrum along the different ray paths. Not accounting for these effects can lead to uncertainties of such magnitude as to yield the final reconstructed dose to be of no practical use.
U.S. Pat. No. 6,438,202 describes a method using a post-patient radiation monitor to verify the entrance radiation and delivered dose. However, this method also does not describe a way to account for the sources of error due to scatter from the patient and an energy dependent detector.
U.S. Pat. No. 6,636,622 describes a method to verify the radiation treatment using the exit-transit dose images. The method described is an iterative process for deriving the primary fluence from the exit images. This process is entirely different from the present invention that does not use an iterative process. The present invention is a different method for arriving at the primary fluence from an exit image. For example, the scatter component of the exit image is never explicitly computed, but accounted for from measured data used in the method.
Methods for reconstructing the dose to the patient from exit images have also been described in the literature. A publication not related to the above cited patents is:
Markus Wendling, Robert J. W. Louwe, a_Leah N. McDermott, Jan-Jakob Sonke, Marcel van Herk, and Ben J. Mijnheer, “Accurate two-dimensional IMRT verification using a back-projection EPID dosimetry method,” Med. Phys. 33, 259-273 (2006), and Markus Wendling, Leah N. McDermott, Anton Mans, Jan-Jakob Sonke, Marcel van Herk, and Ben J. Mijnheer, “A simple backprojection algorithm for 3D in vivo EPID dosimetry of IMRT treatments,” Med. Phys. 36, 3310-3321 (2009). The two reports also uses a series of EPID images for different field sizes and thicknesses as in the present invention, which in the reports is used to estimate an attenuation coefficient and a beam hardening coefficient in a fairly complicated process that is difficult to understand because the report is vague about some steps. Their method performs some conversions on the exit images and the dose to the patient is computed in a back projection algorithm. Whereas the invention here, while using some of the same input data, describes a method to convert the exit image directly to an in air x-ray intensity fluence map, and the dose to the patient is then computed from the in air fluence map using a forward dose algorithm in the same manner typically used in treatment planning systems.
In WD Renner, K Norton, T Holmes, “A method for deconvolution of integrated electronic portal images to obtain incident fluence for dose reconstruction,” JACMP 6, 22-39 (2005), a method is described for converting images of the treatment field taken with an image detector without the patient being there to in air x-ray intensity fluence maps, which are then used to compute the dose to the patient. The method can only be used to reconstruct the dose to the patient from images taken without the patient being there, or from a measurement of the radiation field before it intersects the patient. The method cannot be used to estimate the dose to the patient from measurement of the beam that exits from the patient.